Event Free Survival (EFS) is defined as the time from the date of randomization to the date of an event, where an event is defined as locoregional progression or recurrence, or distant metastasis per Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 as per BICR; Salvage surgery at the primary tumor site; Neck dissection or surgery performed \>20 weeks after completion of Concomitant Chemoradiotherapy (CRT) when invasive cancer is present or Death from any cause.

DRR12 is defined as the proportion of participants with confirmed Complete Response (CR) or Partial Response (PR), and Duration of Response (DOR) lasting greater than or equal to (≥) 12 months, per Response Evaluation Criteria in Solid Tumors, version 1.1 (RECIST 1.1).

mPR rate is defined as the proportion of participants with ≤10% residual viable tumor (RVT) value in the surgical resection sample as determined by local assessment.

cCR12 is achieved when a participant maintains complete clinical response (cCR) as assessed by Independent central review (ICR) for 12 months from the first disease assessment after last dose of study intervention. Timeframe calculation includes 6 months (9 cycles of dostarlimab, with each cycle lasting 21 days) plus an additional 12 months of assessment time, amounting to a total of 18 months.

The progression-free survival is the length of time during and after the treatment of a disease that a patient lives with the disease but it does not get worse.

The time from the date of randomization until first documentation of disease progression, as determined by investigator assessment based on RECIST 1.1, or death due to any cause, whichever occurs first.

Confirmed ORR is defined as the percentage of participants achieving confirmed Complete Response (CR) or Partial Response (PR) per Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 by investigator assessment.

An AE is any untoward medical occurrence in a participant or clinical investigation participant, temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. SAE is defined as any untoward medical occurrence that, at any dose resulted in death, is life threatening, required hospitalization or prolongation of existing hospitalization, resulted in disability/ incapacity, is a congenital anomaly/ birth defect, other situations and is associated with liver injury or impaired liver function. SAEs are subset of AEs. A TEAE is any event that was not present prior to the initiation of study treatment or any event already present that worsens in either intensity or frequency following exposure to study treatment. AEs were coded using the Medical Dictionary for Regulatory Activities (MedDRA) coding system.

An AE is any untoward medical occurrence in a participant or clinical investigation participant, temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. SAE is defined as any untoward medical occurrence that, at any dose resulted in death, is life threatening, required hospitalization or prolongation of existing hospitalization, resulted in disability/ incapacity, is a congenital anomaly/ birth defect, other situations and is associated with liver injury or impaired liver function. SAEs are subset of AEs. A TEAE is any event that was not present prior to the initiation of study treatment or any event already present that worsens in either intensity or frequency following exposure to study treatment. AEs were coded using the Medical Dictionary for Regulatory Activities (MedDRA) coding system.

A DLT is an AE meeting criteria such as, hematologic toxicities of Grade (G) 4 neutropenia/anemia/thrombocytopenia (G3 if bleeding). Non-hematological toxicities include persistent G2 eye events, colitis/diarrhea (G2 unresolved to ≤ G1 within 7 days despite immunosuppressive therapy, G3 for ≥ 72 hours, any G4), G3 pneumonitis, rash (unresolved to ≤ G2 within 2 weeks despite treatment), hypersensitivity/IRR, liver events meeting Hy's Law criteria. G3 toxicity unresolved to ≤G1 or baseline within 3 days with supportive care, or any G4 toxicity. Exclusions include G3 events of electrolyte imbalances correctable within 72 hours without effects, nausea/vomiting/fatigue resolving within 7 days, lymphopenia, and enzyme elevations without pancreatitis. Considerations for DLTs include permanent treatment discontinuation, investigator/sponsor judgment-based events including post-observation period toxicities.

Number of participants with dose modifications (missed doses, dose delays and infusion interruptions) is summarized.

Number of participants with dose modifications (missed doses, dose delays and infusion interruptions) is summarized.

Performance Status was assessed using the ECOG scale (Grades 0-5), where 0: Fully active, able to carry on all pre-disease performance without restriction. Grade 1: Restricted in physically strenuous activity but ambulatory \& able to carry out work of light or sedentary nature; Grade 2 - Ambulatory \& capable of all self-care but unable to carry out any work activities. Up and about more than (\>) 50% of waking hours; Grade 3 -Capable of only limited self-care, confined to bed or chair \> 50% of waking hours; Grade 4 -Completely disabled. Cannot carry on any self-care. Totally confined to bed or chair; Grade 5 -Dead.

Performance Status was assessed using the ECOG scale (Grades 0-5), where 0: Fully active, able to carry on all pre-disease performance without restriction. Grade 1: Restricted in physically strenuous activity but ambulatory \& able to carry out work of light or sedentary nature; Grade 2 - Ambulatory \& capable of all self-care but unable to carry out any work activities. Up and about more than (\>) 50% of waking hours; Grade 3 -Capable of only limited self-care, confined to bed or chair \> 50% of waking hours; Grade 4 -Completely disabled. Cannot carry on any self-care. Totally confined to bed or chair; Grade 5 -Dead

Vital signs including systolic blood pressure (SBP), diastolic BP (DBP), pulse rate (PR) and body temperature (BT) were measured for the participants. DBP: Grade 0 (\<80 millimeters of mercury \[mmHg\]), Grade 1 (80-89 mmHg), Grade 2 (90-99 mmHg), Grade 3 (\>=100 mmHg); SBP: Grade 0 (\<120 mmHg), Grade 1 (120-139 mmHg), Grade 2 (140-159 mmHg), Grade 3 (\>=160 mmHg); PR categories include: 'Decrease to \< 60 beats per minutes \[bpm\]', 'Change to Normal' or 'No Change', and 'Increase to \>100 bpm'; BT categories include 'Decrease to \<=35 degrees Celsius °C', 'Change to Normal' or 'No Change', and 'Increase to \>=38 °C'. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

Vital signs including systolic blood pressure (SBP), diastolic BP (DBP), pulse rate (PR) and body temperature (BT) were measured for the participants. DBP: Grade 0 (\<80 millimeters of mercury \[mmHg\]), Grade 1 (80-89 mmHg), Grade 2 (90-99 mmHg), Grade 3 (\>=100 mmHg); SBP: Grade 0 (\<120 mmHg), Grade 1 (120-139 mmHg), Grade 2 (140-159 mmHg), Grade 3 (\>=160 mmHg); PR categories include: 'Decrease to \< 60 beats per minutes \[bpm\]', 'Change to Normal' or 'No Change', and 'Increase to \>100 bpm'; BT categories include 'Decrease to \<=35 degrees Celsius °C', 'Change to Normal' or 'No Change', and 'Increase to \>=38 °C'. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

Number of participants who received Concomitant medications is summarized.

Number of participants who received Concomitant medications is summarized.

Number of participants with worst-case post baseline (WCPB) from baseline ECG findings is summarized as clinically significant. Data is summarized as Normal, Abnormal - Not Clinically Significant (NCS) and Abnormal - Clinically Significant (CS). Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

Number of participants with worst-case post baseline (WCPB) from baseline ECG findings is summarized as clinically significant. Data is summarized as Normal, Abnormal - Not Clinically Significant (NCS) and Abnormal - Clinically Significant (CS). Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

The QTcF values based on Fridericia formula were rounded to the integer and the values are categorized into the following ranges, inclusively: Grade 0 (\<450 millisecond (msec)), Grade 1 (≥450-≤480 msec), Grade 2 (≥481-≤500 msec), and Grade 3 (≥501 msec). Missing baseline grades were assumed to be Grade 0. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

The QTcF values based on Fridericia formula were rounded to the integer and the values are categorized into the following ranges, inclusively: Grade 0 (\<450 millisecond (msec)), Grade 1 (≥450-≤480 msec), Grade 2 (≥481-≤500 msec), and Grade 3 (≥501 msec). Missing baseline grades were assumed to be Grade 0. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

Number of participants with worst case post-baseline in LVEF from baseline is summarized as 'any decrease (\>0%-\<10% Decrease, 10%-19% Decrease, \>=20% Decrease)', '\>=10% Decrease and \>= Lower limit of normal (LLN)', '\>=10% Decrease and \< LLN', '\>=20% Decrease and \>= LLN' and '\>=20% Decrease and \< LLN' . An increase is defined as an increase in grade relative to Baseline grade. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

Number of participants with worst case post-baseline in LVEF from baseline is summarized as 'any decrease (\>0%-\<10% Decrease, 10%-19% Decrease, \>=20% Decrease)', '\>=10% Decrease and \>= Lower limit of normal (LLN)', '\>=10% Decrease and \< LLN', '\>=20% Decrease and \>= LLN' and '\>=20% Decrease and \< LLN' . An increase is defined as an increase in grade relative to Baseline grade. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

Blood samples were collected for the analysis of hematology parameters and are categorized in alignment with Common Terminology Criteria for Adverse Events (CTCAE) version 5 as Grade 1: mild; Grade 2: moderate; Grade 3: severe or medically significant; and Grade 4: life-threatening consequences. Higher grade indicates greater severity. An increase in grade is defined relative to the Baseline grade. Participants with missing baseline values are assumed to have baseline value of grade 0. Any worst-case post baseline increase in grade along with any increase to a maximum grade of 3 and 4 is summarized. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

Blood samples were collected for the analysis of hematology parameters and are categorized in alignment with Common Terminology Criteria for Adverse Events (CTCAE) version 5 as Grade 1: mild; Grade 2: moderate; Grade 3: severe or medically significant; and Grade 4: life-threatening consequences. Higher grade indicates greater severity. An increase in grade is defined relative to the Baseline grade. Participants with missing baseline values are assumed to have baseline value of grade 0. Any worst-case post baseline increase in grade along with any increase to a maximum grade of 3 and 4 is summarized. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

Blood samples were collected for the analysis of clinical chemistry parameters and are categorized in alignment with Common Terminology Criteria for Adverse Events (CTCAE) version 5 as Grade 1: mild; Grade 2: moderate; Grade 3: severe or medically significant; and Grade 4: life-threatening consequences. Higher grade indicates greater severity. An increase in grade is defined relative to the Baseline grade. Participants with missing baseline values are assumed to have baseline value of grade 0. Any worst-case post baseline increase in grade along with any increase to a maximum grade of 3 and 4 is summarized. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

Blood samples were collected for the analysis of clinical chemistry parameters and are categorized in alignment with Common Terminology Criteria for Adverse Events (CTCAE) version 5 as Grade 1: mild; Grade 2: moderate; Grade 3: severe or medically significant; and Grade 4: life-threatening consequences. Higher grade indicates greater severity. An increase in grade is defined relative to the Baseline grade. Participants with missing baseline values are assumed to have baseline value of grade 0. Any worst-case post baseline increase in grade along with any increase to a maximum grade of 3 and 4 is summarized. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

Blood samples were collected for analysis of clinical chemistry. The summaries of worst-case post baseline (WCPB) from baseline (B) with respect to normal range was analyzed. Data is presented as "XXX B YYY, WCPB YYY", where XXX denotes lab parameter and YYY is high/normal/low. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

Blood samples were collected for analysis of clinical chemistry. The summaries of worst-case post baseline (WCPB) from baseline (B) with respect to normal range was analyzed. Data is presented as "XXX B YYY, WCPB YYY", where XXX denotes lab parameter and YYY is high/normal/low. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits. Worst Case Post-Baseline includes all scheduled and unscheduled visits post baseline.

Urinalysis was performed. Participants with missing value at baseline are assumed to be negative at baseline. All increases are from baseline. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits.

Urinalysis was performed. Participants with missing value at baseline are assumed to be negative at baseline. All increases are from baseline. Baseline is defined as the latest pre-dose assessment with a non-missing value, including those from unscheduled visits.

OS is defined as the time from date of randomization to the date of death, irrespective of the cause of death.

ORR was defined as percentage of participants with a confirmed investigator-assessed Best Overall Response (BOR) of confirmed complete response (CR) or partial response (PR), evaluated using Response Evaluation Criteria in Solid Tumors (RECIST) version (v) 1.1. PR is defined as at least a 30% decrease in the sum of the diameters of target lesions, taking as reference the baseline sum diameters. CR is defined as the disappearance of all target lesions and any pathological lymph nodes (whether target or non-target) must have reduction in short axis to less than (\<)10 millimeters (mm).

Disease-free survival (DFS) is defined as the time from the date of study treatment initiation to the date of first record of any of the following events: Investigator determined: Locoregional progression or recurrence. Distant metastasis. Neck dissection or surgery performed for clinical or radiological disease progression (RECIST 1.1) \> 20 weeks from the end of radiation therapy with tumor present on final pathology. Death due to any cause. Patients not presenting any of the previous events will be censored at the date of last assessment.

An adverse event (AE) is any untoward medical occurrence that occurs in a participant or clinical investigation participant administered a pharmaceutical product, and which does not necessarily have to have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (including clinically significant abnormal laboratory findings), symptom, or disease temporally associated with the use of an investigational product, whether or not considered related to the product. TEAEs defined as any AE either reported for the first time or worsening of a pre-existing event after first dose of study treatment.

Participants with immune related AEs of interest will be assessed.

Blood samples will be collected to assess the following hematology parameters: hemoglobin, Mean corpuscular (MCV), white blood cell count (WBC count), platelets, mean platelet volume, differential WBC count and coagulation factors including International normalized ratio (INR), activated partial thromboplastin time (aPTT) and prothrombin time (PT).

Blood samples will be collected to assess the following chemistry parameters: sodium, potassium, calcium, magnesium, creatinine, bilirubin, alkaline phosphatase, aspartate aminotransferase (AST), alanine aminotransferase (ALT), and albumin.

Number of participants will be assessed.

Number of participants will be assessed.

Participants will be supine or in a semi-recumbent position and rested for approximately 2 minutes before ECGs are recorded.

Concomitant medications will be recorded.

An AE is any untoward medical occurrence that occurs in a participant or clinical investigation participant administered a pharmaceutical product, and which does not necessarily have to have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (including clinically significant abnormal laboratory findings), symptom, or disease temporally associated with the use of an investigational product, whether or not considered related to the product. TEAEs defined as any AE either reported for the first dose of study treatment.

Participants with immune related AEs of interest will be assessed.

Blood samples will be collected to assess the following hematology parameters: hemoglobin, MCV, white WBC count, platelets, mean platelet volume, differential WBC count and coagulation factors including INR, aPTT and PT.

Blood samples will be collected to assess the following chemistry parameters: sodium, potassium, calcium, magnesium, creatinine, bilirubin, AST, ALT, and albumin.

Participants will be supine or in a semi-recumbent position and rested for approximately 2 minutes before ECGs are recorded.

Concomitant medications will be recorded.

An AE is any untoward medical occurrence that occurs in a participant or clinical investigation participant administered a pharmaceutical product, and which does not necessarily have to have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (including clinically significant abnormal laboratory findings), symptom, or disease temporally associated with the use of an investigational product, whether or not considered related to the product. TEAEs defined as any AE either reported for the first dose of study treatment.

Participants with immune related AEs of interest will be assessed.

Blood samples will be collected to assess the following hematology parameters: hemoglobin, MCV, white WBC count, platelets, mean platelet volume, differential WBC count and coagulation factors including INR, aPTT and PT.

Blood samples will be collected to assess the following chemistry parameters: sodium, potassium, calcium, magnesium, creatinine, bilirubin, AST, ALT, and albumin.

Participants will be supine or in a semi-recumbent position and rested for approximately 2 minutes before ECGs are recorded.

Concomitant medications will be recorded.

ORR is defined as the proportion of participants achieving complete response (CR) or partial response (PR) as evaluated by independent blinded central review using RECIST version 1.1.

ORR is defined as the proportion of participants achieving CR or PR as evaluated by independent blinded central review using RECIST version 1.1.

ORR is defined as the proportion of participants achieving CR or PR as evaluated by independent blinded central review using RECIST version 1.1.

ORR is defined as the proportion of participants achieving CR or PR as evaluated by independent blinded central review using RECIST version 1.1.

ORR is defined as the proportion of participants achieving CR or PR as assessed by investigator per irRECIST will be evaluated.

DOR is defined as the time from first documentation of CR or PR by RECIST version 1.1 until the time of first documentation of progressive disease (PD) evaluated using RECIST version 1.1 based on independent blinded central review, or death due to any cause.

DOR is defined as the time from first documentation of CR or PR by RECIST version 1.1 until the time of first documentation of PD evaluated using RECIST version 1.1 based on independent blinded central review, or death due to any cause.

DOR is defined as the time from first documentation of CR or PR by RECIST version 1.1 until the time of first documentation of PD evaluated using RECIST version 1.1 based on independent blinded central review, or death due to any cause.